1. Field of the Invention
The application relates to a process for the production of an optical waveguide preform, in which the cladding layer is applied as a tubular powder layer onto a cylindrical glass core rod and is stabilized by heating in an isothermal temperature field. The sintering of the powder layer to form the cladding finally takes place at temperatures of over 1250.degree. C.
2. Description of the Prior Art
A process of this type is known under the name of MSP Process (MSP=Mechanical Shaping of Preforms) (Article: "Technologies for the Production of Preforms for Optical Fibers", in Elektrisches Nachrichtenwesen Heft 3/4, 1988). Since it is possible, with the presently known processes, to produce optical waveguides from glass in a quality that has almost reached the theoretically possible limit at the attenuation values achieved, intensive work is now being carried out on a reduction of the production costs. This also includes the efforts directed at an increase in the size of preforms, from which optical waveguides of great length can be prepared from one piece. From a preform with an end diameter of fifty-five millimeters (55 mm) and a length of fifty centimeters (50 cm), an optical waveguide with a length of one hundred kilometers (100 km) can be drawn.
If, in the production of preforms by the MSP process, the cladding layer, made of oxide powder with an initial bulk density of between five (5) and fifteen (15) percent, is stabilized by heat treatment to at least twenty five percent (25%) of its final density, where 100 percent density is equal to the density of the final sintered glass material, it has been found that the body is produced with bends and tears. This destruction of the semi-finished preform may be attributed to the interaction of the core and the formed cladding layer acting in opposite directions.
Desirably the stabilized cladding layer must be free of cracks so that the glassy cladding layer sintered from it is also free of cracks whereby the optical waveguides drawn from the finished optical waveguide preform do not show any increase in attenuation.
Since the powdery or strongly porous cladding layer shows a very large radial and axial shrinkage during stabilization, it is desirable to ensure that the tubular cladding layer can slide on the core rod. If this is not achieved, the cladding layer will adhere to the core rod and the optical waveguide preform formed will tear, break or rupture.
The cladding layer, which is still porous during stabilization, contains several substances that would interfere with light guidance in the finished optical waveguide if they were not removed during the stabilization process or thereafter.